In the context of this specification, references to "hobbing" or a gear hobbing process are intended to include those machines and processes which utilize a rotating tool for engaging and forming teeth in a rotating workpiece while the tool and the workpiece are rotated together in a timed relationship. The timed relationship is critical to the formation of teeth of the right geometry and spacing about the perimeter of the workpiece, and the timed relationship is achieved with a drive train which interconnects a spindle for driving the tool with a spindle for driving the workpiece.
Hobbing machines are well known in the art and are utilized for manufacturing spur and helical gears. Typically, such machines include a tool head portion where a cylindrical hobbing tool can be mounted and rotated on its axis while the tool is brought into engagement with one or more workpieces mounted on a workpiece spindle. The workpiece spindle may be designed to carry only a single workpiece or a stack of workpieces, and the machine is provided with mechanisms for feeding the tool into engagement with the workpieces and for then traversing the tool from one end of the stack of workpieces to the other end thereof while the tool and the workpieces are rotated in a timed relationship.
Although bevel gears were used at one time in the part of the drive train that drives the workpiece about its axis (as shown, for example, in U.S. Pat. Nos. 1,543,031, 2,048,503 and 2,704,492), the hobbing machine technology has developed in more recent years to one which utilizes one or more worm drives in the final drive system for providing a rotating moment to the workpiece drive spindle. Examples of worm and wheel drives are shown in U.S. Pat. Nos. 3,232,169 and 3,318,193. The use of work drives developed out of a need for great precision in the final portion of a drive train where there is a requirement to change direction of the power train and to greatly reduce its speed for driving the work spindle of the machine. Worm and wheel drives provide such a dual function of changing direction and reducing speed in the drive train. However, worm drives are costly to manufacture to the standards of precision that are required for present day production machines, and the use of a worm drive limits the speed range available for any given machine. As a result of this, many companies in the business of manufacturing hobbing machines offer two separate lines of machines: a standard machine for use at standard speeds and a special machine with multi-lead worm and worm wheels for use at higher speeds. Alternatively, it is possible to build two separate worm drive mechanisms into a single machine, as shown for example in U.S. Pat. No. 4,286,479. In either case, a customer must invest considerable capital in order to obtain a capacity for operating over a wide range of speeds. In addition to the disadvantages just mentioned, it is also known that worm drives tend to wear excessively and create heat during normal operation. The development of excessive heat in any given area of a machine drive train creates a special problem of maintaining accuracy in the drive train and can lead to hot areas in the machine itself that tend to affect precision alignment of basic machine components which determine the precision relationship between the tool and the workpiece.
In order to provide for a greater range of speeds in a single machine than is possible with worm drives, while, at the same time, obtaining greater efficiency and stiffness in the drive train of the machine than is obtainable with worm gear drives, the present invention utilizes a hypoid gear set in combination with spur gears in the final drive system for a work spindle of a hobbing machine. The hypoid gear set functions to change the direction of the power train (from a horizontal to a vertical axis) and to partially reduce speed at the work spindle, and the spur gears function to further reduce speed at the work spindle. By separating speed reduction into two steps, it is possible to obtain a wider range of work spindle speeds with a given hypoid gear set, and the hypoid gear set does not have to be as accurate (or as costly) as is required for a worm drive of a conventional machine. Hypoid gears offer similar driving characteristics to what is obtained with worm gears because of the number of teeth which are in contact between the hypoid pinion and the hypoid gear which is being driven by the pinion. Also, hypoid drives are relatively quiet and do not create excessive heat. A hypoid drive also offers the possibility of an extended speed range for any given machine, and it is contemplated that a machine utilizing the drive of this invention will be able to operate with work spindle speeds on the order of 120 revolutions per minute or more.
There is a particular advantage in using a hypoid gear set (or even bevel gears) for reverse hand hobbing operations and for a backlash control system which utilizes a supplemental drive train in combination with the main drive train of the machine. It is possible with such a system to preload a substantial portion of the main drive train, so as to remove backlash therefrom, in accordance with the type of cutting being performed by the machine. For example, a positive preload can be used when cutting helical gears with a hobbing tool which is of opposite hand from the helix of the workpiece. Negative preload can be used when the hand of the tool and the workpiece are the same. In both cases, it is possible to effectively stiffen a substantial part of the machine drive train through the use of a supplemental drive.
In a specific embodiment of the invention, the drive train of a hobbing machine is provided with a final drive system for its work spindle characterized by the use of a hypoid pinion mounted to be rotated by a main drive train of the machine and with the hypoid pinion being in meshing engagement with a hypoid ring gear. The hypoid ring gear is operatively connected to the work spindle through an indexing spur gear set which is connected directly to the work spindle. In addition, a supplemental driving system is provided for overcoming load interruptions on the work spindle so as to remove backlash in the gears that are driving the work spindle. This combination of a final drive system utilizing a hypoid gear set with a supplemental driving system also permits an easy adaption of the machine to reverse hand hobbing operations. In addition, the hypoid drive also allows the final indexing gear to be made much larger than the rated diameter capacity of the machine which is a desirable feature.
These and other features and advantages of the invention will become apparent in the more detailed discussion which follows. In that discussion reference will be made to the accompanying drawings as briefly described below.